We will determine how rapidly and in what sequence the hemodynamic and structural (including ultrastructural) characteristics of the pulmonary circulation alter on return to normoxia, comparing male, female, infant and adult rats. We will ascertain whether the hemodynamic and structural response is similar in chronic normobaric and hypobaric hypoxia. Using quantitative morphometric lungs we will study in infant rat lungs the structural and ultrastructural features of the pulmonary vascular bed which have been induced by chronic exposure to hypoxia. The structural effect of chronic hypoxia will be evaluated on the smooth muscle cells in the hilar muscular artery, and on the precursor cells of intrapulmmonary vessels. We will continue our endeavors to establish an electron microscope technique for examination of smooth muscle myosin and to localize tropomyosin and heavy meromyosin.